After I previously talked about problems of input methods for abugida scripts, and added more supporting details to the point, I finally started prototyping possible implementations of the idea (try it out!).
But there are quite a few constraints and tradeoffs that come up once you start thinking about the details. I think these issues apply generally to most abugida scripts. So I am documenting all of the details below. Also, getting a new input method adopted requires more than perfecting just the technical details and user experience — it also requires overcoming user inertia (or creating awareness), and it also requires educating industry experts and those implementing changes. If you have feedback, please send it my way so that I can continue to update this post with the latest information.